CHAMPAIGN, Ill. — More than a dozen agricultural stakeholders, educators, soil and water experts and environmentalists spoke during the Illinois Nutrient Loss Reduction Strategy (NLRS) Annual Partnership Conference, held at the I Hotel and Conference Center in Champaign on Nov. 5. The topic of discussion: recent scientific discoveries regarding statewide streambank erosion-fed nutrient loads entering Illinois waterways. 

While the water remains cloudy for Illinois farmers in terms of attaining NLRS nutrient load reduction goals for nitrogen (N) and nitrates, new technologies used in measuring point and non-point water pollution sources have revealed that agriculture can no longer be identified as a primary contributor to phosphorous (P) loads in the Illinois and upper Mississippi rivers. This is according to Dr. Andrew Margenot, a professor of soil sciences for the University of Illinois, who promised that the next biannual NLRS Report Card, due to be issued in late 2025 or early 2026, will be “fixed” to reflect greater balance assigned to the primary responsibility for streambank erosion P losses to natural sources.

“Streambank erosion is a natural process. There is conflicting evidence on the effect of human activities on (streambank erosion),” said Margenot, who based his conclusions on three years of recent field trials and as many as 80 years of historical evidence. “For example, tile drainage has been found to increase or decrease stream power, which may or may not increase or decrease streambank erosion.”

As a non-point source of streambank erosion P losses, the lion’s share of the total state P “export” into waterways has historically been laid at the feet of Midwest farmers. In truth, much of the P loss accredited to agricultural chemicals can actually be traced to geogenic P, which originates from natural geological sources. Legacy P in waterways can likewise not be blamed primarily on agricultural fields, according to Margenot’s recent studies, with much of it likely forming through natural organic processes in the riverine system. 

“The sediments that are now in the (river) channel will give off P at different times. This introduces a source of variability that’s decoupled from in-field agricultural non-point losses,” Margenot said. “Why does this matter? The current way we break down our P losses in the current NLRS report… incorrectly equates (all) non-point sources with ag. Updates are coming to fix this. We know what is exiting the state as a total, we know what the point sources are discharging by their permits, and so by the difference we can calculate non-point sources. Non-point sources are diffuse and you can not measure them correctly; you have to estimate mathematically (and) not all non-point sources are agricultural.”

Accepting Margenot’s new calculations, the question becomes what percentage of non-point source P losses into Illinois waterways can be fairly attributed to agriculture. “How much is from manure, from fertilizer, from streambanks, etcetera? Some states, like Vermont, have been doing this; they can estimate within the non-point source (category). How can we better finesse that? For the basic reason that the science is stronger, and we can better target resources and conservation dollars towards where the P is actually coming from,” said Margenot.

The general school of thought is that 40 percent of suspended sediments in rivers are thought to be from streambanks, according to the U of I soils expert, with around 31 percent of those sediments thought to be P. “This is global; in some places it is all of (the percentage) and in some places it is none of it. Iowa published an assessment three years ago that found on a concerted estimate and 18-year average how much P is being loaded into streams across the state relative to the amount being exported. They estimated 31 percent, which is what the global evidence suggests on average,” Margenot explained.

The 2023 NLRS Report Card estimated a 35 percent P load in Illinois waterways, with cropland in a negative P balance. This means that in a time of spiking P loads, farmers were using fewer fertilizers on their crops. This suggested to Margenot that there must be other contributing factors contributing to the “historic” P loads the NLRS report suggested were being exported. 

 “That is exactly why these legacy sediments that are encumbered by the process of bank erosion are helpful in helping explain things like this. For this we have a multi-year project where we are trying to understand across the state…what could this magnitude of loading be. We are looking at bank erosion with erosion pins and remote sensing, which lets us go back to the 1930s, albeit somewhat crudely, to form estimates,” he said. 

Margenot’s three-year research effort took samples from some 500 stream sections trying to understand relative rates of erosion and what the P load might be. His findings reveal that the average P content entering streams from streambanks reaches around 20 million pounds per year across Illinois’ 860 river miles. That’s only around 46 percent of the 44 million pound per annum P baseline used by the NLRS. 

Assuming one-fourth of those 20 million pounds of streambank P originates from agriculture, the percentage of P runoff commonly accredited to farmers can be reduced to as little as 29 percent, according to Margenot. Current NLRS apportionments for P in waterways attribute 4 percent to urban runoff, 48 percent to point source and 48 percent to non-point sources including agriculture and streambank erosion. 

“The math doesn’t math at this point,” he concluded. 

The Illinois NLRS is a statewide, collaborative effort working to reduce the amount of nutrients, particularly nitrogen and phosphorus, entering Illinois waterways. Learn more at go.illinois.edu/NLRS.